Conventional magneto-optical (MO) technology employs a two-pass process to erase previously recorded data from an MO disk and write new or updated data onto the disk. A third pass is required to verify the newly recorded data. FIGS. 1a and 1b illustrate the process. During the erase pass (FIG. 1a), data bits 12 previously recorded on a disk 10 are heated by an unmodulated laser beam 14 as they rotate through a field from a bias magnet 16. A combination of the laser heat and the bias field cause all of the bits 12 to assume the same orientation (downward in FIG. 1a), thereby recording zeros and effectively erasing the data. During the next pass (FIG. 1b), the bias field is reversed and the laser beam 14 is modulated to create data bits 18. FIG. 1b includes a plot of the laser power over time as the laser is modulated in pulses between a high power and a quiescent level (very close to off) to change selected erased areas (digital zeros) into recording marks (digital ones). Details of such conventional procedures for erasing and writing data, as well as for reading data, are well known in the art and do not require further elaboration.
It will be appreciated that the two-pass requirement imposes a limit on recording throughput. Consequently, the ability of an MO storage system to perform direct overwrite, without a separate erase pass, has been a desirable goal.
One direct overwrite system which has been suggested includes two adjacent optical heads and two corresponding bias magnets. One head/magnet pair erases data; the other head/magnet pair records data onto the just erased area. The second head/magnet pair is also used for reading data. The cost of manufacturing and aligning such a system to the necessary degree of precision is, however, quite high, a distinct commercial disadvantage.
Another direct overwrite system is known as magnetic-field modulation (MFM) recording in which the laser beam power is maintained at a constant level while the direction of a magnetic field is rapidly modulated. To produce an effective field, the magnetic head should be very close to the recording layer of the MO disk (within several micrometers of the disk surface), a drawback which increases the mechanical complexity of the drive.
Still another direct overwrite system is known as laser-intensity modulation (LIM) recording which uses special, multilayer media and a high power pulse superimposed onto a lower pedestal (or erasing) power. As illustrated in FIG. 2, the disk 20 includes multilayered, exchange-coupled MO films 27 and 28 as active MO layers over a disk substrate. Recording marks 22 are initially formed in one of the layers, the writing or reference layer 28, during a high power portion of the recording waveform under the influence of a normal bias field from the bias magnet 26. The marks are subsequently copied into the overlying memory layer 27 by exchange-coupling during cooling after the recording marks 22 rotate away from the heat of the laser beam 24. The magnetic orientation of the reference layer 28 is reset in the erase direction when the recording marks subsequently pass through an initiating field from an initiating magnet 29, creating a series of digital zeros in the reference layer 28 without affecting the marks copied into the memory layer 27. Included in FIG. 2 is a plot of the laser power over time as new bits are recorded over previously recorded data. The laser power is modulated between the low power level (which leaves the digital zeros) and a high power level (which records digital ones).
The plots in FIGS. 1b and 2 represent pulse position modulation (PPM) recording in which each recording mark represents a digital one (or zero) and the timing positions between the recording marks represent digital zeros (or ones). FIG. 3 is a plot of laser power over time during pulse width modulation (PWM) recording in which digital ones (or zeros) are represented by transitions from low power to high power and from high power to low power. To reduce heat build up in the media, in the waveform illustrated, the mark itself comprises rapid high powered pulses; transitions between immediately adjacent high power pulses are ignored since the resulting recording marks from the individual pulses are intended to overlap each other, forming a single longer recording mark. Zeros (and ones) are represented by timing positions between the transitions.
With increased data densities, and particularly when PWM recording is employed, precise recording mark placement and well defined mark edges are very important. However, an inherent property of multilayered, direct overwrite magnetic materials is that they are two to seven times thicker than magnetic layers employed in conventional, non-overwritable MO media. Moreover, the reference layer lies 90-100 nm from the surface of the overlaying memory layer which absorbs the laser irradiation. Consequently, when recording with short pulses, excessive powers are required, particularly at the higher linear velocities at the outer diameter of the MO disk. Although the use of a pedestal power level improves recording, formation of the magnetic domain in the memory layer is accomplished only at effective media temperatures considerably higher than used on conventional MO media due to the thermal properties of the thick layered materials used in the direct overwrite media. Consequently, blooming and thermal crosstalk effects tend to reduce the quality of the resulting recording marks, and may, in fact, reduce the quality below operating specifications.
The quality of recording marks can be quantified by using several criteria. One such measure is "jitter", which is the standard deviation of the offset of the mark center (in PPM recording) or mark edge (in PWM recording) from the center of the readback clock pulse. The position error normally has a Gaussian distribution which may or may not be centered with respect to the clock pulse center; the standard deviation of the position distribution is the jitter.
A second measure of quality is the "figure of merit" (FOM), which reflects both the average mark position and the jitter of the relevant mark feature (edge or center). FOM is commonly defined as: EQU FOM=(T/2-PS)/Jitter,
where T is the clock period and PS is the average shift in position of the mark feature from the center of the clock pulse.
A third measure of quality is the "bit error rate" (BER), which is the ratio of the number of bits in error to the total number of bits read. The number of bits in error is determined by a bit-by-bit comparison of the data when read back against the original data intended to be recorded.
These measures of mark quality will generally be different with different drives, different media and different drive/media combinations. They can also vary within a drive as the ambient temperature changes, as the disk temperature changes (a disk tends to heat up during extensive write operations), as the drive, and particularly the laser, ages, among other factors. Consequently, no single setting of the write power level will result in optimal mark formation for all drives or even for a single drive in all conditions.